In order to improve the productivity of microalgal lipid Thalassiosira sp. that was grown in the outdoor photobioreactor, in the present study the optimization of N:P:Si content obtained from various sources for growth medium and the optimization of initial inoculum cell concentration were carried out. In a previous study was reported that Thalassiosira sp. produced lipids that could be used as an alternative of biofuel source. However, the effect of cell concentration on the productivity of biomass and lipids remained unknown. In the present study, the N:P:Si ratio derived from a various fertilizer: Medium 1 (mixture of urea-NaH2PO4-metasilicate), Medium 2 (mixture of Hyponex®-metasilicate), Medium 3 (mixture of Hyponex®-SiPPG fertilizer), Medium 4 (mixture of urea-TSP fertilizer-metasilicate), and Medium 5 (mixtute of urea-TSP fertilizer-SiPPG fertilizer) was optimized for the Thalassiosira sp. growth medium. In order to determine the biomass and oil productivity, the optimization of initial inoculum concentration was performed for act of the planting of the outdoor photobioreactor. In addition, iron content in the growth medium was also further optimized. The results showed that the Thalassiosira sp. cells growth in Medium 1 with a salinity of 25 ppt ( ̅ =the average of specific growth rate=0.592±0.107 per day) was the best condition. The following research showed that Medium 2 was the optimum growth medium which produced the average of specific growth rate of 0.703 per day, respectively. The obtained Thalassiosira sp. lipids from the outdoor culture by using Medium 2 with an initial inoculum cell concentration of 0.1, 0.5, and 1.0 ×106 cells/mL was 2.90, 3.04 and 2.86 × 10-4 g/L culture/day, respectively. Optimization result of iron mineral content in the growth medium showed that Medium 2 without additional Fe (0 mg/L FeCl3) was compatible as the medium growth of Thalassiosira sp. with the specific growth rate of 0.569 per day, respectively. Thus, Medium 2 containing 115 mg/L Hyponex® as a source of 11.8 mg/L nitrogen and 2.5 mg/L phosphorous as well as 10 mg/L metasilicate as a source of 1.3 mg/L silicon, with salinity 25 ppt without additional Fe and an initial inoculum cell concentration of 0.5 × 106 cells/mL was the best condition to produce Thalassiosira sp. biomass.